This invention relates to a button (or key) telephone exchange system for interconnecting a local line from a telephone office and one of several telephone sets and for switching the connections between telephone sets by utilizing the office lines.
A prior art key telephone exchange system is constituted by a main device consisting essentially of relay circuits and telephone sets each including mechanical lock buttons or keys connected to the main device with a plurality of interior wiring connections and constructed such that, by the manipulation of the mechanical lock buttons, the mutual connection between the telephone sets on the office line is selected to enable talking between the telephone sets by transmitting an origination signal and receiving a call request signal through an office line, talking power being supplied from the main device. In almost all cases the main device has only the capabilities of establishing a connection to the office line and of supplying the talking power.
For this reason, the number of the wiring connections to respective telephone sets increases with the number of sets, which not only increases the cost of installation but also makes it difficult to maintain the system. Moreover, as the main device is constituted by relays not only does the size become bulky but the reliability of operation also decreases as the number of sets is increased. In addition it becomes extremely difficult to maintain good service, for example control of the origination and call request reservation of the office line, etc.
As an approach for solving these problems, a system has been proposed wherein a control processor is provided in common for the local line and the telephone line for processing information signals originating from and sent to an office line unit and a telephone unit so as to switch the connections as disclosed, for example, in U.S. Pat. No. 4,125,749 issued to Kinoshita et al on Nov. 14, 1978. According to this system, however, since the transmission and reception of all information between the processor and the local line unit and the telephone set unit are made through the same routine, it is necessary to process various information signals and to transmit and receive the information signals, so that where the number of office lines and the number of telephone sets is large, the time necessary for processing all information is increased, thus making quick response of operation impossible.